1 Description #
source: https://leetcode.com/problems/kth-smallest-element-in-a-bst/
Given the root of a binary search tree, and an integer k, return the k^th smallest value (1-indexed) of all the values of the nodes in the tree.
Example 1:
Input: root = [3,1,4,null,2], k = 1
Output: 1
Example 2:
Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3
Constraints:
The number of nodes in the tree is n.
1 <= k <= n <= 10^40 <= Node.val <= 10^4
Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?
2 Solution #
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int kthSmallest(TreeNode* root, int k) {
// Time complexity: O(N)
// Space compleixty: O(1)
preorder(root,k );
return kth_value;
}
void preorder(TreeNode* root, int k){
if(!root){
if(-1 == kth){
kth = 0;
}
return;
}else{
preorder(root->left, k);
kth ++;
if (kth == k){
kth_value = root->val;
}
preorder(root->right,k );
return;
}
}
int kth_value = -1;
int kth = -1;
};